They have been taken to hospital.

The head of the operation to rescue the 12 boys and their coach said on Sunday it was going "better than expected" and would resume on Monday.

The operation to rescue the boys, aged 11 to 16, and their 25-year-old coach by having them dive out of the flooded cave began on Sunday morning, with expert divers entering the sprawling complex for the complicated and dangerous mission.

Shortly before 8pm, Thai navy SEALs, who are taking part in the rescue operation, reported on their official Facebook page that four had been rescued.

Chiang Rai acting govemor Narongsak Osatanakorn, who is heading the operation, said the four boys had been taken to a hospital.

"The operation went much better than expected," Chiang Rai acting govemor Narongsak Osatanakorn said at a news conference, adding that the healthiest of the boys were taken out first. He said the next phase of the operation would start in 10 to 20 hours.

The entire operation to rescue all 13 could last two to four days, depending on weather and water conditions, said army Major General Chalongchai Chaiyakam.

Just after 9pm, Thai navy SEALs posted on their Facebook page again, saying: "Have sweet dreams everyone. Good night. Hooyah."

Narongsak said earlier in the day 13 foreign and five Thai divers were taking part in the rescue and that two divers would accompany each boy as they're gradually extracted.

The divers entered the Chiang Rai cave on an 11-hour round trip to rescue the 12 boys and their soccer coach at 10am on Sunday (1300 AEST).

They planned for two divers to accompany each boy back to the surface through the four kilometres of tight, muddy, water-filled passageways, some of which are less than a metre in diameter.

Six of the divers are with the Australian Federal Police, while another is an anaesthetist from Adelaide along with his diving partner.

A spokesperson for Foreign Minister Julie Bishop said the anaesthetist entered the cave on Saturday to check the boys' health and has been working closely with on-site doctors.

Experienced cave rescue experts consider an underwater escape a last resort, especially with people untrained in diving, as the boys are. The path out is considered especially complicated because of twists and turns in narrow flooded passages.

But Narongsak said earlier that mild weather and falling water levels over the past few days had created optimal conditions for an underwater evacuation that won't last if it rains again.

Before announcing that the rescue was under way, authorities ordered the throngs of media that had gathered at the cave from around the world to leave.

The boys and their coach became stranded when they went exploring in the cave after a practice game on June 23. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.

Australian Associated Press